The use of ultra-high strength steel has been increased in the automotive industry to reduce weight of a vehicle body and to improve safety in a collision. Hot stamping (also called “hot press forming”) has been used as one of ways of forming ultra-high strength steel of 1500 MPa.
The hot stamping is an engineering method of producing an ultra-high strength vehicle body part through a phase transformation to martensite by heating a blank up to an austenite region, press-forming the blank, and then rapidly cooling the blank in a mold.
The hot stamping is often used to ensure strength of crash members such as a center pillar, a roof rail, a bumper, and an impact beam on the vehicle body and to reduce weight by removing reinforcing members.
According to the related art, a flange of a product material formed by hot stamping and a mold are brought in contact with each other first, and thus, heat is removed at the flange first.
Accordingly, the phase transformation to martensite occurs at the flange first, so elongation decreases at the flange, and the material is not smoothly supplied, and thus breaking-off occurs.
Further, when the flange is welded thereafter, the welded portion decreases in strength and thus easily breaks.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may include information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.